How can I get even more cockroaches in my apartment? is a question I’m sure all of you have asked at one time or another. The good news is, all you need to speed up roach production is…other roaches! Or, at least, some bugs about the size of a roach.

The study, published this month in PLoS One, was conducted by Adrienn Uzsák and Coby Schal at North Carolina University…and some lovely German cockroaches. They found that when female cockroaches socialize, they produce eggs faster. And they don’t even have to socialize with other roaches! It just has to be an insect of roughly the same size and shape.

The researchers observed, as so many of us unfortunately have in the past, that cockroaches tend to hang out in groups. Rarely, if ever, do you find just one cockroach in your home. As they say, when it rains, it pours a crapload of cockroaches. In social insects, living in groups has many different effects on the bugs’ behavior and biology. For example, the group dynamic can change how each insect finds and handles food, their choice of mates, and how prepared their body is for baby-making. Cockroaches that are raised in groups even develop faster than roaches raised in isolation. Or, if a group isn’t available, then a good feather-poke can do the trick.

Previous work has already shown that female German cockroaches make eggs faster when in groups, even if the group only consists of other female roaches.
For the study by Uzsák and Schal, they wanted to know when socializing makes the biggest difference (during the day or at night) and why.

German cockroach female showing off her egg case. The result of all that social interaction. Oh, and sex. Photo courtesy of the Public Health Image Library of the CDC.

In the first experiment, the paper showed what was pretty much known already: female cockroaches develop sexually must faster when they have friends. They took the average size of the most developed eggs in the female’s ovaries as a proxy for sexual maturity. Each female has 40 synchronized assembly lines in her ovary. At one end is the most developed egg, at the other, the least developed. The length of the most developed eggs tells you how ready a female is to start pumping out cute little baby roaches. Females who spent 6 days alone had eggs that were more than 0.4mm shorter than females who spent 6 days with a girlfriend.

The researchers were also very interested in whether the time of day spent interacting was important. For this experiment, they tested the two phases of the cockroach day: the scotophase (dark phase) and the photophase (light phase). Roaches are nocturnal, but that doesn’t necessarily mean it’s the time they get the most social interaction. During the day, they’re cooped up with lots of other cockroaches. The researchers thought that getting all that face-time with other cockroaches of the same species during the day might be more important for egg development than bumping into roaches at night.

Surprisingly, the results were the exact opposite. When female cockroaches had even 1 or 2 hours to hang with a girlfriend in the dark, their eggs were bigger than if they had a whole 6 hours together in the light. In fact, the difference between socializing for 6 hours in the light and 6 hours in the dark was the same as the difference between being totally alone for 6 days or having a companion for that whole time. (You might say the difference was night and day. But I would never say that. It’s too cheesy.)

But what is it about being around other cockroaches that gets their ovaries working overtime? Which sense is the most important? Is it sight, sound, smell, taste or touch?

For this paper, the authors tested sight, smell, and touch, but hypothesized that because cockroaches like to touch each other so much (if you’ve ever seen roaches crawling all over each other, you know what I mean), that it would be the most important factor affecting reproduction.

Sight and smell turned out not to be important. Roaches don’t use visual cues for very much, so the fact that being able to see other roaches through a clear petri dish didn’t affect egg size wasn’t surprising. They even did one experiment with two roaches in the same dish, but painted over the eyes of the test roach. Didn’t matter. Blind or not, she still sped up her egg production when someone else was there. But chemical cues, or smells, are widely used by roaches for many aspects of socialization, including mating and courtship. Even so, whether or not isolated females were given a piece of paper drenched in cockroach smell didn’t make any difference in their egg sizes six days later.

What did make a difference was the ability to touch another cockroach. Or a cockroach-like insect. The important thing was that the companion was a: alive (dead roaches didn’t do much for a girl, and neither did moving glass beads) and b: about the size of a cockroach, or bigger. Contact with adult crickets or katydids were about as good as other German cockroaches. On the other hand, ants and beetle larvae didn’t quite do it for them.

The researchers were even able to show that the most important aspect of the interaction was contact with moving antennae. For example, there was this fun experiment: females were either housed alone (solitary) or with just the live and creepy huge antennae of an American cockroach female. To allow the German roach to have contact only with the antennae of the bigger roach, they placed the American roach outside the test female’s petri dish, then cut holes so that those crazy big antennae would stick through. Check out the picture in panel B of this figure:

Figure 5 from Uzsak and Schal.

And sure enough, German roach females that hung out with American roach antennae produced bigger eggs than females kept all alone for the same amount of time. The difference is shown in the bars labeled “isolated” and “untreated” (bar 1). To make sure that the key ingredient was actually the sensation of touching the moving antennae, and not contact with some chemical on them, the scientists carried out several different experiments. The final one was what is shown in the picture in panel C of the figure above. In this experiment the larger roach (the American one) has had its antennae removed and replaced with some artificial fiber (microfibett). As you can see by comparing the bars in the graph labeled 1 (untreated) and 5 (replaced with microfibett), the fake antennae weren’t quite as good at speeding up sexual development in the German roach. But they were way better than nothing at all.

All this science is great, and definitely gives us some great insight into what makes roaches tick. But I mostly want to know what I can do to get rid of them. The best hope from this paper was the finding that females needed to have their own antennae intact to get the benefits of touching other roaches. If a female’s antennae were removed, her eggs were smaller (ie: less developed), especially if she was in contact with another antenna-less female.

Sadly, I don’t think this is a breakthrough in pest control. But the implications for research of social animals are pretty exciting. These experiments were able to separate out the components of socialization that are most important for a specific life trait (reproduction, in this case). They also separated out the giving of social cues from the receiving of social cues. I wish more behavioral biology experiments were set up as carefully as in this paper.

Of course, the best part of this paper for me is how it shows, once again, how everything in biology is connected. Being in contact with another insect’s antennae sets off a series of chemical reactions inside a female cockroach, affecting hormone levels and, eventually, the number of baby cockroaches she’ll make. Unless she gets squashed first.